1. Field of the Invention
This invention relates to automatic shock wave scoring apparatus for scoring the "hit" of a supersonic projectile in a target area where the "hit" is determined from the detection of the acoustic ballistic shock wave. More particularly, this invention relates to the location and configuration of concentric curved acoustic energy conductors (rod sensors) for detecting the ballistic shock wave where the rod sensors form a part of such automatic target scoring apparatus. This invention uses concentric curved acoustic energy conductors to replace the mutually perpendicular linear acoustic energy conductors in the Rohrbaugh U.S. Pat. No. 3,778,059 and replaces the single curved energy conductor in U.S. Pat. Nos. 5,025,424 and 4,351,026 with a pair of concentric curved acoustic conductors which are located only on one side of the target area. Mathematically calculated circles which pass through the score points on the rod sensors and the center of curvature of two pairs of concentric curved acoustic energy conductors intersect in the target area and locate a "hit" when an "off-axis" projectile having a flight path which is not perpendicular to the plane of the target area passes through the target area.
2. Discussion of Prior Art
Two basic methods are employed for determining the "hit" location of a supersonic projectile using the acoustic shock wave generated by the passing projectile which must be fired substantially perpendicular to the plane of the target scoring area. The first employs point sensors arrayed around or near the target area and calculates the projectile's location based on the arrival times of the shock wave at the various sensors. The time of arrival of the ballistic shock wave at each sensor, the locations of the sensors, the speed of sound in air, and the velocity of the projectile are used to make the calculation of the "hit" coordinates. The flight path of the projectile must be substantially perpendicular to the target plane for the "hit" to be determined accurately. Examples of this method of scoring supersonic projectiles are disclosed in U.S. Pat. No. 2,925,582 (Mattei) and U.S. Pat. No. 4,261,579 (Bowyer).
The second method employs petal rod sensors either linear or curved to detect the ballistic shock wave. Linear rod sensors are placed on the sides of the target area as disclosed in U.S. Pat. No. 3,778,059 (Rohrbaugh). The rod sensors replace the point sensors of the first method and determine the projectile's location based on the arrival times of the metal borne shock waves at transducers on the ends of the rod sensors. Only the speed of sound in the metal rods and the time of arrival of the metal borne shock waves at the transducers are needed to determine the "hit" coordinates. As is the case for the point sensor system above, the flight path of the projectile must be substantially perpendicular to the target plane for the "hit" to be determined accurately by the rod sensors.
Since this new invention relates to the improvement of the scoring apparatus which uses rod sensors to score "off-axis" (non perpendicular) projectile flight paths with respect to the target scoring plane, the following is an in-depth discussion of current target scoring which uses the metallic rod sensors.
The automatic gunnery system described in the Rohrbaugh U.S. Pat. No. 3,778,059 detects the ballistic shock wave generated by a supersonic projectile with linear metallic acoustic rod sensors. The rod sensors, one vertical and one horizontal, are located on the perimeter of the target area to be defined. The two rod sensors are mutually perpendicular and describe an XY-coordinate grid within the target area. The ballistic shock wave from a supersonic projectile passing through the grid area impinges on the metallic rod sensors causing secondary acoustic shock waves to be generated in the metallic rods. The secondary shock waves in each rod move away from the impact tangent point where the airborne ballistic shock wave first intercepted the metallic sensor rod and travel in opposite directions toward the ends of the rod. Transducers located on the ends of the rod detect the shock waves in the metal rod and produce electrical signals which start and stop timing circuits. The relative time difference of the start and stop signals from the opposite ends of the rod sensor, coupled with the known velocity of sound in the metallic rod, allows calculation of the impact point on the linear rod. Each rod sensor, one vertical and one horizontal, acts independently of the other to produce one axis of the XY-coordinate grid which defines the target area. The intercept of the XY-coordinates defines the "hit" for a projectile which passes through that intercept point. The resulting "hit" data are displayed as digital numbers representing the distance from the target center or displayed in graphic form as a dot on a display screen.
Curved rod sensors as described in U.S. Pat. Nos. 5,025,424 and 4,351,026 function in a like manner to the linear rod sensors but define the XY-coordinates of a "hit" as the intersection of two radial lines, where each radial line passes through the center of curvature and the tangent score point of a curved rod sensor. At least two curved rod sensors are placed on the same side of the target area to produce radial lines which intersect in the target area and define the "hit" coordinates. Both curved rods are placed below the line of fire and away from possible impact damage.
A serious limitation of both the linear rod system and the curved rod system is the requirement that the flight path of the projectile be substantially perpendicular to the target plane for accurate determination of the "hit" coordinates. The large errors introduced when "off-axis" projectiles are scored using an acoustic shock wave system can be mathematically determined and the errors corrected provided the impact velocity and flight path angle of the projectile are known in advance, which is not normally the case for typical shooting range applications, especially if either the target or shooter is moving.
A limitation of the linear rod sensor system described in the Rohrbaugh invention, U.S. Pat. No. 3,778,059, is the necessity for the vertical rod sensor to define one axis of the scoring grid in the target area. The vertical rod sensor is visible to the shooter which makes camouflage of the target area very difficult when the target scoring system is employed for training with pop-up type targets. Also, the vertical rod sensor is in the line of fire and can be damaged unless protected by armor plate. Such armor plate is unacceptable in most applications because of the weight and safety considerations from ricocheting bullets.